This invention relates generally to methods and apparatus for reconstructions of images in computed tomography (CT), and more particularly to methods and apparatus for maintaining artifact free images with high spatial resolution for normal dose scans while providing adequate correction for low dose scans.
In at least some computed tomograph (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a post patient collimator for collimating scattered x-ray beams received at the detector. A scintillator is located adjacent the post patient collimator, and photodiodes are positioned adjacent the scintillator.
Multislice CT systems are used to obtain data for an increased number of slices during a scan. Known multislice systems typically include detectors generally known as 3-D detectors. With such 3-D detectors, a plurality of detector elements form separate channels arranged in columns and rows. Each row of detectors forms a separate slice. For example, a two slice detector has two rows of detector elements, and a four slice detector has four rows of detector elements. During a multislice scan, multiple rows of detector cells are simultaneously impinged by the x-ray beam, and therefore data for several slices is obtained.
Because different CT applications require different image quality, physicians may use very low scan techniques for certain CT applications such as low dose lung screening, and CT localization scans for PET applications. In such applications, high image spatial resolution can be traded off with the lower scan dose. However, organ boundaries should be well delineated, high-density blood vessels and tumors should be visualized and streaking artifacts should be minimized.
At least some known adaptive pre-smoothing algorithms used in normal dose applications fail to provide adequate correction for these extremely low dose applications. In the CT localization scans for PET applications a post-smoothing operation may be performed on images to obtain certain noise characteristics. However, such post-smoothing may introduce residual streaking artifacts and over-smoothness when the scans are acquired with low CT dose.